Condensed Matter Physics Commons^™

Multiscale Examination And Modeling Of Electron Transport In Nanoscale Materials And Devices, Douglas R. Banyai

Dissertations, Master's Theses and Master's Reports - Open

For half a century the integrated circuits (ICs) that make up the heart of electronic devices have been steadily improving by shrinking at an exponential rate. However, as the current crop of ICs get smaller and the insulating layers involved become thinner, electrons leak through due to quantum mechanical tunneling. This is one of several issues which will bring an end to this incredible streak of exponential improvement of this type of transistor device, after which future improvements will have to come from employing fundamentally different transistor architecture rather than fine tuning and miniaturizing the metal-oxide-semiconductor field effect transistors (MOSFETs) …

Understanding Electronic Structure And Transport Properties In Nanoscale Junctions, Kamal B. Dhungana

Dissertations, Master's Theses and Master's Reports - Open

Understanding the electronic structure and the transport properties of nanoscale materials are pivotal for designing future nano-scale electronic devices. Nanoscale materials could be individual or groups of molecules, nanotubes, semiconducting quantum dots, and biomolecules. Among these several alternatives, organic molecules are very promising and the field of molecular electronics has progressed significantly over the past few decades. Despite these progresses, it has not yet been possible to achieve atomic level control at the metal-molecule interface during a conductance measurement, which hinders the progress in this field. The lack of atomic level information of the interface also makes it much harder …

Synthesis, Integration, And Physical Characterization Of Graphene And Carbon Nanotubes, David P. Hunley

Theses and Dissertations--Physics and Astronomy

Graphene and carbon nanotubes are among the hottest topics in physics today. Both materials exhibit numerous remarkable mechanical, electrical, optical, and thermal properties that make them promising materials for use in a large number of diverse applications, especially in the field of nanotechnology. One of the ultimate goals driving the fields of nanoscience and nanotechnology has been the attainment of atomically precise construction of intricate integrated systems consisting of materials with diverse behavior. Specifically, it is desirable to have high performance conductors, semiconductors, and insulators integrated into complex atomically precise arrangements. This dissertation represents the culmination of work that has …

A Study On Atomically Thin Ultra Short Conducting Channels, Breakdown, And Environmental Effects, Abhishek Sundararajan

Theses and Dissertations--Physics and Astronomy

We have developed a novel method of producing ultra-short channel graphene field effect devices on SiO₂ substrates and have studied their electrical transport properties. A nonlinear current behavior is observed coupled with a quasi-saturation effect. An analytical model is developed to explain this behavior using ballistic transport, where the charge carriers experience minimal scattering. We also observe multilevel resistive switching after the device is electrically stressed. In addition, we have studied the evolution of the electrical transport properties of few-layer graphene during electrical breakdown. We are able to significantly increase the time scale of break junction formation, and we …

The Development And Implementation Of Systems To Study The Physical Properites Of Tantalum Trisulfide And Small-Molecule Organic Semiconductors, Hao Zhang

Theses and Dissertations--Physics and Astronomy

The charge-density-wave (CDW) material orthorhombic tantalum trisulfide (TaS₃) is a quasi-one dimensional material that forms long ribbon shaped crystals, and exhibits unique physical behavior. We have measured the dependence of the hysteretic voltage-induced torsional strain (VITS) in TaS₃, which was first discovered by Pokrovskii et. al. in 2007, on temperature and applied torque. Our experimental results shows that the application of torque to the crystal could also change the VITS time constant, magnitude, and sign. This suggests that the VITS is a consequence of residual torsional strain originally present in the sample which twists the polarizations …

Strategies For Increasing The Néel Temperature Of Magnetoelectric Fe2Teo6, Sai Mu, Kirill D. Belashchenko

Department of Physics and Astronomy: Faculty Publications

Ways to increase the Néel temperature T_N in the magnetoelectric Fe₂TeO₆ antiferromagnet are explored with the help of first-principles calculations. Substitution of larger ions like Zr or Hf for tellurium increases the superexchange angles. The compensating O vacancies tend to form bound complexes with Zr dopants, which do not degrade the electronic band gap. T_N is estimated to increase by 15% at 12.5% Te → Zr substitution with such compensation. Substitution of N for O is favorable due to the decreased charge-transfer gap. The overall effect for N³⁻ substitution compensated by O vacancies …

Multiferroic Tunnel Junctions And Ferroelectric Control Of Magnetic State At Interface, Y. W. Yin, M. Raju, W. J. Hu, John D. Burton, Y.-M. Kim, A. Y. Borisevich, S. J. Pennycook, S. M. Yang, T. W. Noh, Alexei Gruverman, X. G. Li, Z. D. Zhang, Evgeny Y. Tsymbal, Qi Li

Alexei Gruverman Publications

As semiconductor devices reach ever smaller dimensions, the challenge of power dissipation and quantum effect place a serious limit on the future device scaling. Recently, a multiferroic tunnel junction (MFTJ) with a ferroelectric barrier sandwiched between two ferromagnetic electrodes has drawn enormous interest due to its potential applications not only in multi-level data storage but also in electric field controlled spintronics and nanoferronics. Here, we present our investigations on four-level resistance states, giant tunneling electroresistance (TER) due to interfacial magnetoelectric coupling, and ferroelectric control of spin polarized tunneling in MFTJs. Coexistence of large tunneling magnetoresistance and TER has been observed …

Electrostatic Charge On Flying Hummingbirds And Its Potential Role In Pollination, Marc Badger, Victor Manuel Ortega-Jimenez, Lisa Von Ribenau, Ashley Smiley, Robert Dudley, Alexei Gruverman , Editor

Alexei Gruverman Publications

Electrostatic phenomena are known to enhance both wind- and insect-mediated pollination, but have not yet been described for nectar-feeding vertebrates. Here we demonstrate that wild Anna's Hummingbirds (Calypte anna) can carry positive charges up to 800 pC while in flight (mean ± s.d.: 66 ± 129 pC). Triboelectric charging obtained by rubbing an isolated hummingbird wing against various plant structures generated charges up to 700 pC. A metal hummingbird model charged to 400 pC induced bending of floral stamens in four plants (Nicotiana, Hemerocallis, Penstemon, and Aloe spp.), and also attracted falling Lycopodium spores at distances of < 2 mm. Electrostatic forces may therefore influence pollen transfer onto nectar-feeding birds.

Magnetic Soft X-Ray Tomography Of Magnetic Swiss Roll Architectures, Robert Streubel, F. Kronast, P. Fischer, O. G. Schmidt, D. Makarov

Robert Streubel Papers

A further increase in performance of state-of-the-art spin-electronics can be achieved by either miniaturizing its functional components or harnessing the third dimension. The first route, however, faces physical limitations as properties of nanostructures and their response to external stimuli can drastically change in reduced dimensions. An alternative way is to go from planar 2D structures to 3D architectures [1]. Such 3D functional elements can be obtained for example by rolling up initially planar strained thin films into Swiss roll like objects with multiple windings (Fig. 1a). A major advantage of this technology platform is the possibility to fabricate compact multifunctional …

Magnetization Pumping And Dynamics In A Dzyaloshinskii-Moriya Magnet, Alexey Kovalev, Utkan Güngördü

Department of Physics and Astronomy: Faculty Publications

We formulate a phenomenological description of thin ferromagnetic layers with inversion asymmetry where the single-domain magnetic dynamics experiences magnon current-induced torques and leads to magnon-motive forces. We first construct a phenomenological theory based on irreversible thermodynamics, taking into account the symmetries of the system. Furthermore, we confirm that these effects originate from Dzyaloshinskii-Moriya interactions from the analysis based on the stochastic Landau-Lifshitz-Gilbert equation. Our phenomenological results generalize to a general form of Dzyaloshinskii-Moriya interactions and to other systems, such as pyrochlore crystals and chiral magnets. Possible applications include spin current generation, magnetization reversal and magnonic cooling.

Analytical And Numerical Studies Of Photo-Injected Charge Transport In Molecularly-Doped Polymers, Amrita Roy Chowdhury

Doctoral Dissertations

"The mobility of photo-injected charge carriers in molecularly-doped polymers (MDPs) exhibits a commonly observed, and nearly universal Poole-Frenkel field dependence, µ ~ exp [square root (ß₀E)], that has been shown to arise from the correlated Gaussian energy distribution of transport sites encountered by charges undergoing hopping transport through the material. Analytical and numerical studies of photo-injected charge transport in these materials are presented here with an attempt to understand how specific features of the various models developed to describe these systems depend on the microscopic parameters that define them. Specifically, previously published time-of-flight mobility data for the molecularly …

Surface Photovoltage Transients For P-Type Algan, Karen L. Phumisithikul

Theses and Dissertations

There is an understanding of surface photovoltage (SPV) behavior for GaN, yet little is known about the SPV behavior for AlGaN. In this work, a Kelvin probe was used to measure the SPV for p-type AlGaN. Very slow SPV transients were found in AlGaN, which could not be explained with a simple thermionic model. A possible explanation of this behavior is the segregation of impurities to the surface, which causes significant reduction of the depletion region width (down to 2 nm), with carrier tunneling and hopping becoming the dominant mechanisms responsible for the SPV transients. To verify this assumption, …

Silicon Nanostructures For High Capacity Anodes In Lithium Ion Batteries, Tyler M. Selden

Theses and Dissertations

In this study we looked at several different silicon nanostructures grown for the purpose of optimizing anodes for lithium ion batteries. We primarily focused on two distinct types of structures, nanospirals, and Rugate structures. The samples were designed to have the mechanical robustness to endure the massive expansion caused by lithiation of silicon. All of the samples were grown using an electron beam evaporator. Scanning electron microscope images show that we have achieved the desired structural growth. The spirals were shown to have an average diameter of 343 nm on polished copper, and 366 nm on unpolished copper. The Rugate …

Ferroelectric Polarization Dependent Interface Effects, Xiaohui Liu

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Utilization of the switchable spontaneous polarization of nanometer scale ferroelectric materials offers a promising avenue for future nanoelectronic devices. In this dissertation, we use density-functional calculations and phenomenological modeling to explore the effects of interface termination on thin-film heterostructures, the effects of electron doping in bulk ferroelectric materials on ferroelectric stability, and the effects of ferroelectric polarization switching on the electronic and transport properties of interfaces.

For SrRuO₃/BaTiO₃/SrRuO₃ epitaxial heterostructures grown on SrTiO₃, we find that the built-in dipole at the BaO/RuO₂ terminated interface leads to a strong preference for one polarization. …

C-Metrics In Gauged Stu Supergravity And Beyond, H. Lu, Justin F. Vázquez-Poritz

Publications and Research

We construct charged generalizations of the dilaton C-metric in various fourdimensional theories, including STU gauged supergravity as well as a one-parameter family of Einstein-Maxwell-dilaton theories whose scalar potential can be expressed in terms of a superpotential. In addition, we present time-dependent generalizations of the dilaton C-metric and dilaton Ernst solutions, for which the time evolution is driven by the dilaton. These C-metric solutions provide holographic descriptions of a strongly-coupled three-dimensional field theory on the background of a black hole, a gravitational soliton, and a black hole undergoing time evolution.

Vibrational And Structural Signatures Of The Crossover Between Dense Glassy And Sparse Gel-Like Attractive Colloidal Packings, Matthew A. Lohr, Tim Still, Raman Ganti, Matthew D. Gratale, Zoey S. Davidson, Kevin B. Aptowicz, Carl P. Goodrich, Daniel M. Sussman, A. G. Yodh

Physics & Engineering Faculty Publications

We investigate the vibrational modes of quasi-two-dimensional disordered colloidal packings of hard colloidal spheres with short-range attractions as a function of packing fraction. Certain properties of the vibrational density of states (vDOS) are shown to correlate with the density and structure of the samples (i.e., in sparsely versus densely packed samples). Specifically, a crossover from dense glassy to sparse gel-like states is suggested by an excess of phonon modes at low frequency and by a variation in the slope of the vDOS with frequency at low frequency. This change in phonon mode distribution is demonstrated to arise largely from localized …

Ferroelectric Tunnel Junctions With Graphene Electrodes, Haidong Lu, Alexey Lipatov, Sangjin Ryu, D. J. Kim, H. Lee, M. Ye. Zhuravlev, Chang-Beom Eom, Evgeny Y. Tsymbal, Alexander Sinitskii, Alexei Gruverman

Evgeny Tsymbal Publications

Polarization-driven resistive switching in ferroelectric tunnel junctions (FTJs)—structures composed of two electrodes separated by an ultrathin ferroelectric barrier—offers new physics and materials functionalities, as well as exciting opportunities for the next generation of non-volatile memories and logic devices. Performance of FTJs is highly sensitive to the electrical boundary conditions, which can be controlled by electrode material and/or interface engineering. Here, we demonstrate the use of graphene as electrodes in FTJs that allows control of interface properties for significant enhancement of device performance. Ferroelectric polarization stability and resistive switching are strongly affected by a molecular layer at the graphene/BaTiO₃ interface. …

Electrostatic Breakdown Analysis, Sam Hansen

Senior Theses and Projects

Materials potentially suitable for spacecraft construction were exposed to electrostatic discharge in the USU Materials Physics Group lab, with hopes of identifying samples that possess greater resistance to breakdown. Breakdown shape and size may be important to determining material suitability for spacecraft construction [1]. The discharge damage sites of tested samples were examined, measured and logged into a matrix file for data analysis. Once logged, data was sorted within the matrix and compared graphically to identify trends.

Electrostatic Breakdown Analysis, Sam Hansen, Jr Dennison, Allen Andersen

Senior Theses and Projects

Materials potentially suitable for spacecraft construction were exposed to electrostatic discharge in the USU Materials Physics Group lab, with hopes of identifying samples that possess greater resistance to breakdown. Breakdown shape and size may be important to determining material suitability for spacecraft construction [1]. The discharge damage sites of tested samples were examined, measured and logged into a matrix file for data analysis. Once logged, data was sorted within the matrix and compared graphically to identify trends.

Nonlinear Optimal Filter Technique For Analyzing Energy Depositions In Tes Sensors Driven Into Saturation, Benjamin Shank, Jeffrey Yen, Blas Cabrera, John Mark Kreikebaum, Robert Moffatt, Peter Redl, Betty Young, Paul Brink, Matthew Cherry, Astrid Tomada

Faculty Publications

We present a detailed thermal and electrical model of superconducting transition edge sensors(TESs) connected to quasiparticle (qp) traps, such as the W TESs connected to Al qp traps used for CDMS (Cryogenic Dark Matter Search) Ge and Si detectors. We show that this improvedmodel, together with a straightforward time-domain optimal filter, can be used to analyze pulses well into the nonlinear saturation region and reconstruct absorbed energies with optimal energyresolution.